So, as Kaffe is licensed under the GPL, the combination of Kaffe +
Classpath must be licensed under the terms of the GPL (as per Kaffe's
license). No exception for linking (a la Classpath) may be allowed.

But that's not the question at hand. The question is whether you are
allowed to run a GPL-incompatible application A on kaffe+libs. Given the
following debian-legal view:

"Precisely, they supported an interpretation of derivative work or not
being defined as whether app A would work with B only or with a
variety of Bs from different vendors."

this means that A running on kaffe+libs is not a derivate work of
kaffe+libs provided that it also work on a variety of jvm+libs (for
instance Sun's, IBM's, gij and sablevm+classpath). The licensing of
keffa+libs itself is irrelevant.

Now, when a debian source package only works with kaffe, and this
package is GPL incompatible, the independence of the app from kaffe is
far from obvious, specially if you are a user running on a platform

where no JDK is available from Sun.

This might hold if kaffe is the only jvm working on that architecture
(Sun has no specific status here). So in particular if sablevm and gij
do not work on that arch, and there is no port of Sun's JDK. Does such
an architecture exist?